Several configurations of Hybrid Electric Vehicles (HEV) systems are known. As used herein, a HEV system generally propels a vehicle in several modes such as using an internal combustion engine only, a dynamo or combination motor/generator only, or by combining torques from both the engine and the dynamo. FIG. 1 illustrates an example of a HEV system 100 that includes an engine, two dynamos or motor/generator combinations—MG1 and MG2, two inverters—INV1 and INV2, and a planetary gear set PG.
The engine and MG1 cooperate via the planetary gear set to provide the main motive power to the vehicle. In one mode, MG1 generates electric power to react against engine torque. This interaction produces a reactionary torque to the sun gear that is transmitted to vehicle wheels. The variable speed relation of MG1 to the engine determines the final speed of the vehicle and so may be viewed as an electric continuously variable transmission (CVT). In another mode MG1 produces torque to start the engine. In another mode MG1 produces electrical energy to operate the vehicle's electrical system or be stored in a battery. When the battery cannot accept the continuous MG1 generator power, MG2 can consume this power to produce additional propulsion torque to the wheels. In another mode known as regenerative braking, MG2 can generate electricity to be stored in the battery for future use. The various modes of operation will now be described in more detail.
Engine start: To start the engine, power is applied to MG1 to act as a starter. Because of the size of the motor generators, starting the engine requires relatively little power from MG1. Furthermore, the engine is not equipped with a conventional starter motor and so typical starter motor sound is not heard. Engine start can occur while the vehicle is stopped or moving.
Forward travel: In general, the engine and motor are operating cooperatively to propel the vehicle. When accelerating at relatively low speeds, the engine may operate at a speed greater than the wheels and drive MG1 operating as a generator. The electric energy output by MG1 is fed to MG2, and torque from MG2 operating as a motor is added to the engine torque applied to the vehicle driveshaft. When cruising at relatively high speed, the engine may operate at slower than the wheels but still drives MG1 operating as a generator to producing electrical power that is fed to MG2. Again, MG2 operates as a motor to combine with the engine torque to propel the vehicle. During steady, relatively normal speed operation the engine generally provides all of the power to propel the car. Then during conditions when torque demand exceeds engine capability, such as during heavy acceleration or driving up a steep incline at high speed, electrical energy from the battery is consumed to make up the difference. Whenever the required propulsion power changes, the battery quickly balances the power budget, allowing the engine to change power relatively slowly.
Reverse travel: In some HEV systems there may not be a conventional gearbox to provide a reverse gear. To back the vehicle the computer may feed negative voltage to MG2 and so provide negative torque to the wheels. However, there have been instances when backing vehicles up steep hills was not possible because of insufficient torque.
Silent operation: At slow speeds and moderate torques the HEV may operate without running the internal combustion engine at all. Electricity may be supplied only to MG2, while allowing MG1 to rotate freely and thus decoupling the engine from the wheels. This mode is popularly known as “Stealth Mode.” Provided that there is enough battery power, the car can be driven in this silent mode for some miles without consuming gasoline. The battery may be later recharged by plugging the system into a battery charging system or an electrical outlet in a parking garage.
Neutral gear: Most jurisdictions require automotive transmissions to have a neutral gear that decouples the engine and transmission. The neutral gear can be provided by turning the electric motors off. In this mode the planetary gear is stationary if the vehicle wheels are not turning. If the vehicle wheels are turning, the ring gear will rotate causing the sun gear to rotate as well. The engine inertia will generally keep the carrier gear stationary unless the speed is large. In this mode the MG1 freewheels so no power is dissipated.
Regenerative braking: By drawing power from MG2 and depositing it into the battery pack, the HEV can decelerate the vehicle while saving electrical power for future use. Typically, the regenerative brakes in an HEV system absorb a significant amount of the normal braking load, so the conventional brakes on HEV vehicles may be undersized compared to brakes on a conventional car of similar mass.
Compression braking: The HEV system may have a special transmission setting labeled ‘B’ for Brake that may be manually selected in place of regenerative braking to provide engine braking on hills. Also, if the battery is approaching potentially damaging high charge levels, the HEV system may switch to conventional compression braking, drawing power from MG2 and shunting it to MG1, thereby speeding the engine with throttle closed to absorb energy and decelerate the vehicle.
Battery charging: The HEV system may charge its battery without moving the car by running the engine and extracting electrical power from MG1. The power gets shunted into the battery, and no torque is supplied to the wheels.
The system described above relies on two dynamos (i.e. motor/generator combination) and two inverters to operate or propel a vehicle. These dynamos and invertors are relatively expensive. What is needed is a HEV system that does not require all of these parts to provide the modes of operation described above and so is less expensive to manufacture.